Advances in plasma processing have facilitated growth in the semiconductor industry. In the competitive semiconductor industry, a manufacturer may gain a competitive edge if the manufacturer has the ability to maximize throughput and/or to produce quality devices at a lower cost. One method for controlling throughput is to control the dechuck sequence to optimize the substrate-release time.
During substrate processing, a substrate is usually clamped to a lower electrode (such as an electrostatic chuck). Clamping may be performed by applying a direct current (DC) potential to the lower electrode to create an electrostatic charge between the substrate and the lower electrode. To dissipate the heat on the substrate during substrate processing, an inert gas (such as helium) may be applied through various channels in the lower electrode to the backside of the substrate to improve the thermal heat transfer between the substrate and the lower electrode. Consequently, due to the helium pressure on the substrate, a relatively high electrostatic charge is required to clamp the substrate to the lower electrode.
Once substrate processing has been completed within the processing chamber, a dechuck sequence is performed in which the clamping voltage is turn off. Even though the clamping voltage is set to zero, a residual electrostatic force remains due to the electrostatic charge between the substrate and the lower electrode. To discharge the electrostatic charge between the substrate and the lower electrode, a low density plasma may be generated to neutralize the attraction force between the substrate and the lower electrode. Once the electrostatic charge has been removed, lifter pins disposed within the lower electrode may be raised to lift the substrate upward to separate the substrate from the surface of the lower electrode, thereby allowing a robot arm to remove the substrate from the plasma processing chamber.
If the electrostatic charge has not been satisfactorily removed, partial sticking may exist resulting in partial substrate hinging to the surface of the lower electrode, thereby causing part of the substrate to break apart when the lifter pins is pushed upward from the lower electrode. The partial sticking may not only damage the substrate but the debris caused by substrate cracking may also require the plasma processing system to be taken offline for chamber cleaning.
In addition, if the electrostatic charge has not been satisfactorily discharged, enough charge may still exist on the substrate to cause arcing between the substrate and the robot arm when the robot arm tries to remove the substrate from the processing chamber. Arcing is an uncontrolled event that may cause undesirable results, such as damage to devices on the substrate and/or the robot arm.
Additionally and/or alternatively, a small voltage biased in the opposite charge of the clamping voltage may be applied to the lower electrode to facilitate dechucking. In an example, if the clamp voltage is 10 volts, then a voltage charge of −1 volt may be applied to the lower electrode during the dechuck sequence. The application of a clamped voltage in the opposite charge causes the positive charge to flow toward the negative charge to aid in the neutralization of the electrostatic force between the substrate and the lower electrode.
Given that the processing environment may vary depending upon the type of processing system, the type of processing modules, the substrate structures, the recipe, and the likes, the time period for executing a successful dechuck sequence may vary. Since the application time period is unknown beforehand and the consequences for improper dechucking are severe, the tendency is to apply the dechuck sequence for a conservatively long specified time period in order to ensure that there is sufficient time for the electrostatic charge to be sufficiently discharged. Unfortunately, both methods of dechucking (at zero volts and at a bias voltage of reverse polarity) still do not always provide a safe and efficient manner of releasing the substrate.
In some cases, the electrostatic charge may be such that only a minimal amount of time is required to discharge. However, the specified time period method does not provide an early-detection method for identifying when the substrate may be safely removed from the lower electrode. As a result, throughput is negatively impacted as time is wasted while the unhinged substrate remains in the processing chamber for the entire specified time period before the unhinged substrate may be removed from the chamber. Also, the existence of the dechuck plasma in the processing chamber for the additional (and unnecessary) time may also contribute to the premature degradation of the chamber components and/or unwanted etching of the substrate.
In other cases, the electrostatic charge may not have been sufficiently discharged after the specified time period has elapsed. As a result, the attempt to remove the hinged substrate may cause the substrate to break. Even if the substrate does not crack, the remaining residual electrostatic charge on the substrate may cause the pneumatic lift mechanism to exert a large force on the lifter pins in order to separate the substrate from the lower electrode. Accordingly, the force exerted on the substrate may cause the substrate to shift away from the process center, thereby causing the substrate to be improperly aligned for the next recipe step. In addition, the residual electrostatic charge on the substrate may cause arcing between the substrate and the robot arm, thereby causing damage to the devices on the substrate and/or the robot arm.
Instead of just executing the dechuck sequence for a specified period of time, certain mechanical parameters (such as inert gas flow, inert gas pressure, and lifting pin force) may be monitored to aid in determining when a substrate may be deemed to have been separated from the lower electrode. In an example, if the inert gas flow (e.g., helium gas flow) to the backside of the substrate exceeds a predetermined threshold, the electrostatic charge is considered to be sufficiently discharged and the substrate may be removed from the processing chamber. In another example, if the inert gas pressure falls below a predetermined threshold, the electrostatic charge is considered to be discharged. Likewise, if the lifting pin force falls below a predetermined threshold value, the substrate is considered to be sufficiently discharged. However, if any of the threshold values is not traversed, then the electrostatic charge is deemed to have been insufficiently discharged and the mechanical forces and/or the bias voltage/current in the opposite charge may be adjusted.
However, the aforementioned methods tend to be time consuming and cumbersome. For example, in one case, only one or two parameters may be adjusted at any one point in time since adjusting too many parameters at once may cause an uncontrolled dechuck sequence.
Since the amount of electrostatic charge due to the clamping voltage may vary depending upon a number of factors (such as the type of lower electrode, the recipe, the process module, and the like), a high degree of variability may exist. Given the high degree of variability, monitoring based on mechanical values (such as helium flow, induced pressure, and or force of lifter pins) is insufficient in optimizing the dechuck sequence since the mechanical values do not accurately and/or adequately characterize the actual electrostatic charges between the substrate and the lower electrode. In an example, one of the mechanical values (such as inert gas flow, inert gas induced pressure, and/or lifter pin force) indicates that a predetermined threshold value (the value that has been designated at which the substrate may be safely released from the lower electrode) has been traversed; however, the electrostatic charge may be nonuniform across the surface of the substrate. Thus, isolated pockets may exist in which the electrostatic charge has not been sufficiently removed. As a result, isolated hinging may still occurs resulting in damage to the substrate when the substrate is separated from the lower electrode.
In addition, since none of the monitored mechanical values accurately characterizes the actual electrostatic charge between the substrate and the lower electrode, a residual amount of charge may still exist on the substrate even though the substrate may be successfully lifted from the lower electrode. As a result, arcing may still occur between the substrate and the robot arm, resulting in damages to devices on the substrate and/or the robot arm.
In view of the foregoing, there are desired improved techniques for optimizing the dechuck sequence.